Immune privilege is an immunological phenomenon whereby some cells, tissues and organs fail to invoke a full immune response to foreign antigens, such as present in foreign cells, tissues, organs and pathogens. Traditionally, this phenomenon has had great impact on tissue transplantation. For example, foreign tissue grafts placed in immune privileged sites (e.g. anterior chamber of the eye, cornea, brain, testis, pregnant uterus) enjoy extended, often indefinite, survival, whereas similar grafts placed at sites that are not immune privileged (e.g. skin, kidney capsule) are rapidly rejected. Similarly, grafts prepared from immune privileged tissues experience extended, often indefinite, survival when implanted at body sites which normally reject foreign tissue.
Immune privilege is granted to the eye because the retina contains a complex matrix of tissue and cells, some of which are post-mitotic, which may be damaged if exposed to the full weight of the systemic immune and inflammatory responses. Immune privilege is maintained by the physical barrier of epithelial cells layers, and also by the active expression of immunosuppressive and anti-inflammatory factors by ocular cells.
This tradeoff between possible collateral damage and disease control is maintained except under extraordinary circumstances such as a serious physical injury or an infection which could potentially threaten the life of the host. Under these conditions the immune privilege status can be modified to allow to all or part of the systemic immune and inflammatory responses access into the eye. For example, it has been reported that retinal laser burn (RLB) not only causes damage to the eye but also causes a loss of immune privilege in both the burned and unburned eye (Qiao et al., 2009, Am. J. Pathol. 174 414).